Abstract:
Cooling-dehumidification technology is gradually applied to the semi-closed structure of the greenhouse. The reason can be that the indoor air humidity is usually higher than the suitable growth range of the planted crops. Among them, the airside performance of the fin-tube heat exchangers directly dominates the efficiency of the cooling-dehumidification system. In this study, a systematic investigation was made to clarify the influence mechanism of the greenhouse environment on the airside performance of the aluminum fin-tube heat exchangers with the hydrophilic coating (thickness 0.8 μm). An airside performance experiment was then conducted on the greenhouse environment created by a wind tunnel, in which the air temperature was 285 to 308 K and the relative humidity was 60% to 90%. The experimental platform of the wind tunnel consisted of the airflow rate measurement, the air temperature and humidity treatment, the fin-tube heat exchanger test, and the refrigerant circulation circuit, according to the American Society of Heating Refrigerating and Air conditioning Engineer (ASHRAE) standards. A comparison was made on the airside performance of the exchanger with and without hydrophilic coating. The correlation analysis was carried out between the predicted and experimental data, in terms of the heat transfer, mass transfer, and friction factor for those without hydrophilic fins. It was found that the predicted values deviated significantly from the experimental data. By contrast, multiple nonlinear regression was utilized to add the correction factors of temperature and relative humidity into the correlations of the fin-tube heat exchanger without hydrophilic coating. The new correlation was then applied to the fin-tube heat exchangers with the hydrophilic coating in the greenhouse environment. The results showed that the hydrophilic coating dominated the airside performance of the fin-tube heat exchanger in the greenhouse environment. The heat transfer factor, mass transfer factor, and friction factor of the fins with the hydrophilic coating were smaller than that of those without hydrophilic coating, in which the hydrophilic coating posed the most significant effect on the friction factor. The maximum difference in the friction factor of fin-tube heat exchangers with and without hydrophilic coating was 53.9%, indicating that the fins with the hydrophilic coating effectively promoted the discharge of condensate. The heat transfer factor, mass transfer factor, and friction factors of the fins with the hydrophilic coating decreased with the increase of airside Reynolds number and refrigerant inlet temperature, but increased with the rise of relative humidity. The mass transfer factor was more sensitive to the inlet relative humidity. Once the inlet temperature of refrigerant was 285 K, the inlet air relative humidity was 80%, and the air inlet temperature increased to 300 K, indicating the maximum heat transfer factor. There was no influence of the air inlet temperature on the heat transfer performance of the airside in this case. The predicted values of the selected correlation of the smallest deviation were larger than the experimental data. The maximum deviation reached 52.5%, 219.6%, and 71.1% for the heat transfer factor, mass transfer factor, and friction factor, respectively. The new correlations for the corrected heat transfer factor, mass transfer factor, and friction factor were 92.9%, 96.4%, and 96.4% of the experimental data within ±10%, after the correction factors were introduced for the temperature and relative humidity into the selected correlations. The mean deviation of the heat transfer factor, mass transfer factor, and friction factor were 5.1%, 5.9%, and 4.7%, respectively. The finding can provide a strong reference for the thermal design and application of fin-tube heat exchangers with the hydrophilic coating in the cooling-dehumidification systems of the greenhouse.